Duct Booster Fans improve airflow to under performing air registers in rooms hard to heat or cool. This may be due to undersized air duct or excessive air duct runs. Duct Booster Fans can also be used as an In-line Exhaust Fan or Fresh Air Intake Fan. They may be used with Flexible Air Duct or Metal Duct. Duct Booster Fans are available for Round Duct or Flat Duct. Room Register Booster Fans for weak floor registers can increase airflow up to 80%.